Method and apparatus for tool orientation in a bore hole

ABSTRACT

An orienting device for a gun or jet perforator or the like to permit setting at a selected angle to a ferrous element such as an adjacent casing string, comprising an exciter coil producing an alternating electromagnetic field and a pair of receiver coils longitudinally spaced from the exciter coils, the disposition of the receiver coils being such that the voltages induced therein vary differentially with the angle presented by the ferrous element by reason of the distortion of the otherwise axially symmetrical field. Electronic means are provided to convert the differential voltages to a pulsed signal which is received at the surface and caused to register the orientation angle. Motor means are provided to rotate the device. All operating power, control signals, and information signals are transmitted by a single conductor cable serving also to suspend the device.

United States Patent Estes et al.

[54] METHOD AND APPARATUS FOR TOOL ORIENTATION IN A BORE HOLE [72]Inventors: James D. Estes; Kirby De Gough,

both of Houston, Tex.

[73] Assignee: N L Industries, Inc., New York,

[22] Filed: May 6, 1971 [21] Appl. No.: 140,813

3,032,107 5/1962 Rumble et al.... .....175/4.5l

3,175,608 3/1965 Wilson ..l75/4.5l 3,307,626 3/1967 Bielstein... 166/2553,307,642 3/1967 Smith ..l75/4.5l 3,342,275 9/]967 Mellies ..175/4.5l

& am Hi i Dec. 5, 1972 Primary Examiner-David H. Brown Attorney-DelmarH. Larsen, Robert L. Lehman, Fred Floresheimer and Roy F. House [57]ABSTRACT An orienting device for a gun or jet perforator or the like topermit setting at a selected angle to a ferrous element such as anadjacent casing string, comprising an exciter coil producing analternating electromagnetic field and a pair of receiver coilslongitudinally spaced from the exciter coils, the disposition of thereceiver coils being such that the voltages induced therein varydifferentially with the angle presented by the ferrous element by reasonof the distortion of the otherwise axially symmetrical field. Electronicmeans are provided to convert the differential voltages to a pulsedsignal which is received at the surface and caused to register theorientation angle. Motor means are provided to rotate the device. Alloperating power, control signals, and information signals aretransmitted by a single conductor cable serving also to suspend thedevice.

15 Claims, 11 Drawing Figures PATENTEDHEB 5 m2 SHEET 3 [IF 4 Era .9.

INVENTOR5. JAM65 0- 557 55 [/EBV P660U6H METHOD AND APPARATUS FOR TOOLORIENTATION IN A BORE HOLE This invention relates to an apparatus andprocedure for orienting a tool, such as a perforator, in a bore hole,such as an oil well, and more particularly in a well bore which containstwo or more casing strings in a side-byside relationship.

For many operations carried out by means of tools lowered in bore holes,often to great depths, it is essential to be able to determine theorientation of the tool when emplaced at the selected depth. Many toolsare lowered to depth on a cable, and it will be readily understood thatduring the course of lowering the tool, the latter may rotateconsiderably, so that its orientation can only be determined withcertainty by some device operating at the depth of the tool itself.

This problem is particularly acute in the case wherein an oil or gaswell is completed so as to permit production from more than one zone.Such multiple zone completions are frequently carried out by running twoor more strings of casing in a side-by-side relationship into' a singlewell bore which penetrates all the zonesin question. In order to producea particular zone, a gun perforator is run through one of the strings ofcasing and placed opposite the zone to be produced. The gun in such acase is of the type which fires all ofits bullets or jets in a singledirection, and these bullets or jets must be directed so that the othercasing strings or strings will not be perforated. In this manner, eachzone to be produced is perforated from a different casing string, sothat it is possible to produce each zone independently of the others. Itwill be appreciated that in order to do this with certainty and safety,the orientation of the perforating tool must be known just beforefiring, with respect to the other casing strings.

A useful review article on the subject appears in World Oil, Feb. 1,1962, pages 52-59, by R. W. Scott.

Representative prior art solutions to the problem outlined above are US.Pat. No. 3,426,850, to McDuffie; US. Pat. No. 3,426,849, to Brumble; andUS. Pat. No. 3,426,851, to Arendt.

The McDuffie patent describes a method of orienting a gun perforatorunder the general conditions already outlined, in which selected stringsof casing contain radioactive material which is used as an orientationtarget by a radiation detector used in conjunction with a perforatinggun. The practical disadvantage here is that the strings not beingperforated must be prepared in advance with a radioactive substance.

The Brumble patent shows a multiple line system wherein one lineoperates a perforating gun designed to fire in only one direction andalso contains a directed source of radiation, while radiation detectorsare operated by other line or lines in the other strings of casing. Thepractical disadvantage of this system is that it requires running wirelines into all casing strings.

The Arendt patent shows a single line system wherein the line controls atool containing a perforating gun designed to tire in only onedirection, a source of radiation, and a collimated radiation detector.The practical disadvantage here is that the resulting density readingsmay be seriously affected by the presence or absence of cement aroundthe casings as well as bythe distance between the casings.

An object of the present invention is to provide an apparatus andprocedure for orienting a tool such as a gun perforator designed toshoot in only onedirection in a bore hole such as an oil well whichcontains two or more casing strings in a side-by-side relationship, sothat the perforator may be fired in a desired direction so as to safelyand reliably clear the other casing strings which are not desired to beperforated, and moreover to accomplish this by an essentiallyelectromagnetic means, free of the hazards associated with radioactivematerials and capable of a high degree of precision.

Another object of the invention is to provide such an apparatus andprocedure that may be operated from the surface on a single conductorwire line.

Other objects of the invention will appear as the description thereofproceeds.

In the drawings,

FIG. 1 is a general view, combining a schematic representation of theabove-ground installation as well as an enlarged cross-sectional view ofthe bore hole and casings at the depth of the perforator assembly. FIGS.2 and 3 are cross sections taken at the approximate depth of theperforator assembly showing typical arrangements with respectively twocasing strings and three casing strings in a single bore hole.

FIG. 4 is a block diagram showing the circuits which may be used in theorientation device.

FIGS. 5, 6, 7, and 8 show different possible arrangements of receivercoils used in the invention.

FIG. 9 shows an optional checking arrangement for use with theinvention.

FIG. 10 shows a still further optional checking arrangement for use withthe invention.

FIG. 11 supplements FIG. 4 in showing-wave forms at various points ofthe circuit.

Generally speaking, and in accordance with illustrative embodiments ofthe invention, we provide an orienting device which for exemplary andillustrative purposes is preferably and most conveniently unitized withthe perforating device as a single tool for lowering into the bore hole,and more particularly into the casing to be perforated. The orientingdevice comprises an exciter coil which produces an electromagnetic fieldwhich in an isotropic embodiment is symmetrical about its axis. Spacedlongitudinally from the exciter coil, either above or below but in ourpreferred embodiment below is a receiver means. In the preferredembodiment, two receiver coils are disposed therein, one of which,conveniently termed a reference coil, is preferably although nonecessarily disposed coaxially with the tool and therefore with the borehole, while the other coil, which may for convenience be termed a sensorcoil, is disposed at an angle with respect to the axis of the tool. Aswill be described in detail below, means are provided to energize theexciter coil and to detect the voltages induced in the reference andsensor coils, and means are also provided to rotate the orienting devicetogether with the perforating gun so that a favorable orientation may beselected and achieved prior to firing the gun.

Coming now to a detailed description of the invention as depicted invarious embodiments in the drawings, FIG. 1 shows a vertical crosssection of a bore hole 10 which contains two casing strings, 11 and 12.Both strings are cemented in the bore hole, sections of the cement beingshown at 13, 14, and 1S. Suspended in the bore hole by a cable 16 is aunitized tool comprising a belly spring section 17 which maintains anyselected position of the tool in'the hole, and also functions as acentralizer; a motor section 18 which serves to rotate the balance ofthe tool therebelow as desired and controlled from the surface; anexciter coil means 19; an electronic section 20; a receiver means 21,functioning as a non-symmetrical electromagnetic field detector; andfinally the device 22, the orientation of which is to be adjustablycontrolled, and which in our preferred embodiment is a perforating gun,two bullet portions thereof being shown at 23 and 24. In the dispositionshown in FIG. 1, the perforating gun has been oriented so that when thebullet sections 23 and 24 are fired, the perforations will be formed inthe formation 25 to be produced without the bullets or jets havingdisturbed the integrity of the other casing string 12.

FIG. 1 also shows in schematic form hoist means 27 and a power andrecording instrument means 28.

FIG. 2 is a section taken horizontally through FIG. 1 just above thetool itself, showing the top of the belly spring section 17 in place incasing string 11, in side-byside relationship with the second casingstrip 12.

As mentioned, there may be several casing strings instead of just two,and FIG. 3 shows three such casing strings for an arrangement in allother respects essentially similar to that of FIGS. 1 and 2.

Reverting now to FIG. 1, cable 16 is of the conventional type used inwire line operations of the type concerned, and thus not requiringdetailed description. It comprises a steel cable strong enough tosupport the assembly, in the interior of which is an insulated copperconductor serving to supply electrical power to the tool and also toconvey the electrical signals to the surface.

The belly spring section 17 is again of conventional construction,containing belly or bow springs serving primarily to maintain the toolat whatever point in the hole at which it has been positioned by theoperator. A secondary function is to centralize the tool in the hole.

The exciter coil means 19, which as already men tioned may be rotated atwill by motor section 18, may be simply a coil which when energizedcauses an alternating electromagnetic field to be formed in thesurrounding region which is symmetrical about the axis of the device andaccordingly symmetric about the axis of the casing string 11, by virtueof the centralizing action of the belly spring section 17, for thecondition of an isotropic environment. Thus, the configuration of theelectromagnetic field surrounding exciter coil 19 is independent of therotation thereof within the casing string. However, in actual use, thesurrounding medium is not isotropic, but is strongly anisotropic byreason of the pressure of the additional casing string 12 as shown inFIGS. 1 and 2 or indeed several such strings as shown in FIG. 3. Thus,when exciter coil 19 is energized, the electromagnetic field in thesurroundings, which ,of course includes the casing strings as well asthe bore hole and the surrounding formation, is severely distorted fromwhat would otherwise be axial symmetry by the presence of the additionalcasing string or strings. Moreover, as already mentioned, the particularconfiguration of the field in a given case, such as that illustrated inFIG. 1, remains essentially unchanged as motor section 18 rotates thebalance of the tool including the exciter coil 19 and the receiver means21. The departure from symmetry occasioned by the fact that the device22 may have some unilateral bullet portions 23 and 24 is quitenegligible.

Coming now .to the receiver means 21, in the preferred embodiment thiscomprises two receiver or pickup coils 30 and 31 as shown in FIG. 5, inwhich the exciter coil portion of means 19 is shown as 26. Alternativeconfigurations for the pair of receiver coils are shown in FIGS. 6, 7,and 8, wherein the coil pairs are 32 and 33; 34 and 3S; and 36 and 37respectively.

By way of general explanation, it may be noted that the receiver coils30 and 31 and like pairs as in the other FIGS. 6, 7, and 8, are disposedin one of any number of configurations, all of which are characterizedby the fact that, first, both coils act as pickup coils in that theyrespond to the alternating electromagnetic field produced by excitercoil 26; second, that under completely isotropic conditions, that is, inthe absence of any ferromagnetic material such as a second casing stringwhich would cause the electromagnetic field produced by exciter coil 26to depart from axial symmetry, the voltage induced in each coil isindependent of rotation of the pair of coils about the axisof thedevice; third, that in the presence of a distorting element such as anadjacent second casing string, the ratio of the voltages produced in thetwo coils of any given pair will change as the device is rotated aboutits axis.

Thus, for example, considering the configuration of FIG. 5, if thisdevice is centrally disposed in a single casing string and thusin anisotropic environment, substantially equal voltages will be induced inboth coils 30 and 31, assuming that they are of like construction, thisassumption being made merely to simplify the-explanation. Moreover, asthe device is rotated about the axis shown in FIG. 5, the voltagesinduced in both coils will remain equal, independent of the azimuth. Onthe other hand, suppose that a second casing string is alongside thedevice on the right hand side thereof as the figure is viewed. Underthis condition, coil 31 will have a higher voltage induced in it thanthat of coil 30 because of the effect of the ferromagnetic casing stringin concentrating the lines of force on that side. If under this sameassumed condition, the device of FIG. 5 is rotated through then thevoltages induced in coils 30 and 31 will be approximately equal; and ifrotation is continued for an additional 90, thus bring coil 30 closestto the second casing string and coil 31 farthest away, then coil 30 willhave a higher voltage induced therein than coil 31.

It is convenient to consider one of each pair of receiver coils as areference coil, and the second of the pair as a sensor coil. Thus, inFIGS. 5, 6, 7, and 8, it is convenient to regard coils 30, 32, 34, and36 respectively as the reference coil of each pair, and coils 31, 33,35, and 37 respectively as the sensor coils.

It will be clear that even for a fixed separation of exciter coil andreceiver coils, the voltage induced in the latter will be subject toconsiderable variation, both in amplitude and phase, as caused forexample by varying casing diameter, casing wall thickness, proximity tocasing collars, and the like. However, our arrangement avoids anydifficulty on this score, since we use a pair of coils and utilize theratio of the voltage induced in the .lvmm mun two coils and moreoverregister this ratio in the form of a selected function of angularrotation with respect to the axis of the device.

From the foregoing, it will be clear how the variation in relativevoltage arises in the receiver coil pair for the other embodiments shownin FIGS. 6, 7, and 8. In FIGS. 6 and 7, reference coils 32 and 34produce a voltage which is substantially independent of rotation, whilesensor coils 33 and 35 are highly dependent upon their orientation withrespect to the adjacent casing string or strings because the axes ofthese two coils 33 and 35 are set at a considerable angle from thevertical axis of the device itself. The variation in the relativevoltages of the two receiver coils in the device of FIG. 8 as the deviceis rotated follows the same explanation as already given in the case ofFIG. 5.

It will be apparent that a wide choice of dimensions, configurations,and the like is possible for the exciter and receiver coils, and in anyspecific design these will be dictated by sound engineeringconsiderations. Thus, exciter coil 26 being essentially a power sourcemay be wound with relatively heavy gauge wire over an ironwire core ofsubstantial length, such as 12 -inches. While the exciter coil may beoperated over 'a wide range of frequencies, we prefer to drive it atabout 60 Hz., at a power consumption of to watts. The receiver coilsfunction essentially to develop voltage, and may be conveniently andpreferably wound with much finer wire and a correspondingly largernumber of turns, and on a much shorter core, such as l to 2 inches. Theydo not necessarily have to be identical; we prefer to have the referencecoil about twice as large as the sensor coil. A convenient and preferredspacing is to place the pair of receiver coils 24 inches from the centerof the exciter coil 26.

Returning to the overall view as shown in FIG. 1, we find it convenientto energize the electronic section 20, the motor section 18, and, whenrequired, the perforating device 22 by direct current. 40 volts ofapplied voltage operates only the electronic section; when the voltageis increased to 70 volts, the direct current electric in motor section18 is activated to rotate that portion of the tool below it. Theperforating device may be fired when desired by applying a voltageexceeding 100 volts, which may simply be a pulse as short as 100milliseconds. The arrangement of the electrical circuits so as todifferentially respond to these varying voltages is conventional and sowell known in the art that it need not be described in detail here.

The signals from the two pickup coils and 31 and the like may becompared in any desired fashion to ascertain which orientation of thetool gives a peak response for the sensor coil. A system which we findconvenient and which we prefer, although others will occur to thoseskilled in the art, is shown in block outline in FIG. 4, and in somewhatmore detail in FIG. 11.

Referring to FIG. 7, this shows the relative positions of exciter coil26, reference coil 34 and sensor coil 35, although it will be clear thatthe description applies to other selected dispositions of the pickupcoils, as indicated for example in FIGS. 5, 6, and 8. As indicated inFIG. 11, the signal from 'coil 34 is split, a portion going to referenceamplifier 40 and a part to phase shifter 41. The latter signal is mixedwith the signal in mixer'42. The typical wave form for the signal fromreference coil 34 is shown at 43, as a typical sine wave. As previouslyexplained, this is independent of the orientation of the tool. Thetypical wave forms of the signal from sensor coil 35 are shown at 44 forthree different conditions of operation. That designated 1 is for thecondition of no external pipe being present. As previously explained,the electromagnetic field produced by exciter coil 26 under thiscondition is axially symmetrical, so that there is no induced voltage insensor coil 35 but merely a low amount of noise. The second wave formshown at 44 is for the condition of the'tool aimed at the external pipe,and shows the induced sinisoidal voltage. The third wave form shown at44 is for the condition of the tool aimed directly away from the pipe,which gives rise to a like voltage exactly 180 out of phase from thesecond condition. The effect of the phase shifter is shown at 45.-

The mixed signal leaving mixer 42 has one of three wave forms, for. thethree conditions discussed, depending upon the amplitude and phase ofthe signal from coil 35, giving the results shown at 46. This is fed toamplifier 47, the output of which is a square wave, the phase of whichis proportional to the ratio of the voltage in coil 35 to that of coil34. The wave form for the three conditions discussed is shown at 48.This signal is fed to gate 49, which is opened by the negative part ofthe square wave 50 produced by amplifier 40. Only positive parts of thesignal from amplifier 47 which occur at the same time as the negativeparts of the reference signal from amplifier 40 are passed on tointegrator 51. Thus, gate 49 functions as a phase detector producing apulse, the width of which is proportional to the phase differencebetween the reference signal from amplifier 40 and the sensor signalfrom amplifier 47, as indicated by the three typical wave forms shown at52. The pulses are integrated in integrator 51 to give a DC voltagewhich is proportional to the cosine of the angle of the tool with theexternal casing, as indicated at 53. This voltage output from 51 isapplied to voltage-controlled pulse generator 54, the output of which isa 50 microsecond, 40 volt pulse, the rate of which is controlled by theinput voltage, as indicated at 55. These pulses are fed to the linedriver 56, and applied to the wire line, where they are detected at thesurface in unit 28 as shown in FIG. 1. In this unit, the pulses areseparated from the exciter voltages present in the line, and areamplified and counted and their integrated value is used to drive arecording pen on a strip chart recorder, all in accordance with meansknown to those skilled in the art. The maximum deflection registered atthe surface then corresponds to the azimuthal location of the externalstring or strings of casing.

It is entirely possible and indeed we prefer to use a single conductoras already described for line 16, since the applied DC voltages, thealternating voltage for driving the exciter coil, and the pulse outputfrom electronic section 20 do not mutually interfere with each other andmay readily be sorted out as needed by means known in the art.

Occasionally, the down-the-hole conditions may be so complex and obscurethat it may be desirable to run a check on the operation of the device.For example, there may be as many as five strings of casing in asideby-side relationship, or the separation between the casings may beextreme. Two means of making such a check are shown in FIGS. 9 and 10.

"UN", Anna In FIG. 9, the device in casing string 11 is as previouslydescribed. However, for this check, the exciter coil 26 is shut off, andan auxiliary battery-operated exciter unit 60, comprising a batterysection 61, an alternator section 62, and an exciter coil 63 is loweredinto casing string 12 on a separate wire line 64. It will be clear thatthis auxiliary exciter device 60 will produce an alternatingelectromagnetic field strongly asymmetrical with respect to the pickupcoils in casing string'll, so that the orientation of the latter may bedoubly checked. At the same time, a check on the depth of the pickupcoils is given by noting how far the device 60 must be lowered on itswire line 64 to achieve maximum response. The device and procedure ofFIG. 9 has a further utility in that it may be used to orient theperforator in cases where the bore holes contain casing strings ofnon-ferrous metal, such as aluminum alloys, or of plastic.

A second type of check is shownin FIG. 10 in which the tool in casingstring 11 is as previously described. In this procedure, the excitercoil is operated as usual, but when a check is desired, a length oftransformer iron 70, which may for example be approximately 1 inch indiameter and 2 feet long, or like ferromagnetic'device, is lowered onits wire line 71 into casing string 12. When it is opposite the tool incasing string 11, and more particularly when it is oppositeapproximately the midpoint between the exciter and'pickup coils, it willgreatly increase the asymmetry of the electromagnetic field alreadybrought about by the presence of casing string 12.

Reverting now to our invention broadly, it will be seen that itaccomplishes its objects. In addition to the advantages alreadydiscussed, it may be further mentioned that a serious limitation inprior art density system discussed earlier is its unreliability whenused in small diameter tubing, disposed within substantially largercasing, such as often occurs. The device of the present invention can beproduced in various diameters with excellent reliability, and isoperative even under the condition just mentioned.

Our invention has been described in terms of our preferred embodiment,in which the electromagnetic field produced by the exciter coil, andmore particularly any asymmetry in this field, is detected by receivermeans 21 in the form of two pickup coils. It is entirely possible to useother types of receiver means 21 capable of detecting theelectromagnetic field and responsive to any asymmetry thereof. Forexample, magnetometers of various types, such as those based on the Halleffect, flux gate magnetometers and rubidium vapor magnetometers may beused. Of course when the receiver means 21 embodies a magnetometer ofthe type which is primarily responsive to a steady state magnetic field,it should be used with appropriate circuitry so as to be responsive tothe essentially alternating electromagnetic field involved in thepresent invention. Devices of this type are well known and have beenhighly developed both as to miniaturization and sensitivity. All suchreceiver means 21, including those of the coil type describedhereinabove in detail, may be termed electromagnetic field sensorsresponsiveto the magnitude and the configuration of the electromagneticflux.

We wish it to be understood that we do not desire to be limited to theexact details of construction shown and described, for obviousmodifications will occur to a personskilled in the art.

Having described the invention, we claim:

1. A device for sub-surface emplacement in a bore hole adjacent to aferrous element for determining the orientation of said element withrespect to said device comprising:

an exciter coil producing an electromagnetic field in the regionsurrounding said coil, said field having axial symmetry in the absenceof said element but having axial asymmetry in the presence of saidelement; reference coil longitudinally spaced from said exciter coil andadapted for the production of an induced voltage from said field; sensorcoil adjacent said reference coil likewise adapted for the production ofan induced voltage from said field, said sensor coil being positionednon-symmetrically with respect to the longitudinal axis of said excitercoil; means for rotating said device in said bore hole; means fordetermining the ratio of said induced voltage in said sensor coil tosaid induced voltage from said reference coil as a function of theangular orientation of said device with respect to said ferrous element.2. A device in accordance with claim l wherein said reference coil iscoaxial with said exciter coil and said sensor coil is positioned at anangle to the axis of said exciter coil and said reference coil.

3. A device in accordance with claim 1 wherein said reference coil andsaid sensor coil are spaced from the axis of said exciter coil and eachhas its axis parallel to the axis of said exciter coil.

4. A device in accordance with claim 1 wherein said reference coil andsaid sensor coil are each spaced from the axis of said exciter coil andwherein each has its axis positioned at a like angle from said excitercoil axis.

5. An orientatable perforating device for subsurface use and adapted toperforate in a preselected orientation comprising, in combination:

positioning means;

rotating motor means attached to said positioning means;

exciter coil means likewise attached to said positioning means, saidexciter coil means producing an electromagnetic field which is axiallysymmetrical with respect to said device when in an isotropicenvironment;

receiver means longitudinally spaced from said exciter coil means androtatable by said motor means, said receiver meanscomprising anelectromagnetic field sensor responsive to the magnitude and theconfiguration of said electromagnetic field, said sensor providing anelectrical signal indicative of said magnitude and said configuration;

means for transmitting said signal to the surface;

perforating means actuatable from said surface and carried by saiddevice and rotated by said motor means.

1 (Ann mm" 6. A device in accordance with claim in which said receivermeans comprises a reference coil and a sensor coil, each adapted for theproduction of an induced voltage from said field.

7. A device in accordance with claim 5 in which said sensor is amagnetometer.

8. An orientatable perforating device for subsurface use and adaptedto'perforate in a preselected orientation comprising, in combination,positioning means;

rotating motor means attached to said positioning means; exciter coilmeans likewise attached to said position ing means, said exciter coilmeans producing an electromagnetic field which is axially symmetricalwith respect to said device when in an isotropic environment; receivercoil means longitudinally spaced from said exciter coil means androtatable vby said rotor means and including a reference coil capable ofproducing an induced voltage from said electromagnetic field and asensor coil means capable of producing an induced voltage from saidfield, said sensor coil means having its axis oriented at an angle tothe longitudinal axis of said device so that when said field is axiallynon-symmetrical, the voltage induced in said sensor coil varies with itsrelative orientation with respect to said non-symmetrical field;

means responsive to the ratio of the induced voltage in said sensor coilto the induced voltage in said reference coil and producing a signalindicative of said ratio which is transmitted to the surface;

perforating means actuatable from said surface and carried by saiddevice and rotated by said motor means.

9. A method for orienting an actuatable subsurface device in a bore holecontaining a first casing string and a second casing string, to apreselected orientation 10 comprising:

positioning in said first casing string a source of electromagnetic fluxnormally axially symmetric with said first casing string butasymmetrically distorted by the presence of said second casing string;

positioning a rotatable receiver means longitudinally spaced from saidsource within said first casing string, said receiver-means beingresponsive to the magnitude and the configuration of said electromagnetic flux and providing an electrical signal indicative of saidconfiguration; transmitting said signal to said surface;

rotating said receiver means so as to cause a registration at saidsurface of its orientation with respect to said second casing string;

rotating said sub-surface actuatable device into said preselectedposition;

subsequently actuating said device.

10. The method in accordance with claim 9 wherein said device is aperforator.

11. The method in accordance with claim 9 wherein said source ofelectromagnetic flux is placed in said second casing string, and saidrotatable receiver means is not necessarily longitudinally spaced fromsaid source.

12. The method in accordance with claim 9 wherein said receiver meanscomprises a reference coil capable o producing an induced voltage fromsaid electromagnetic flux and a sensor coil means capable of producingan induced voltage from said flux, said sensor coil means having itsaxis oriented at an angle to the longitudinal axis of said casingstring.

13. The method in accordance with claim 12 wherein said device is aperforator.

14. The method in accordance with claim 9 wherein said receiver means isa magnetometer.

15(The method in accordance with claim 14 wherein said device is aperforator.

1. A device for sub-surface emplacement in a bore hole adjacent to aferrous element for determining the orientation of said element withrespect to said device comprising: an exciter coil producing anelectromagnetic field in the region surrounding said coil, said fieldhaving axial symmetry in the absence of said element but having axialasymmetry in the presence of said element; a reference coillongitudinally spaced from said exciter coil and adapted for theproduction of an induced voltage from said field; a sensor coil adjacentsaid reference coil likewise adapted for the production of an inducedvoltage from said field, said sensor coil being positionednon-symmetrically with respect to the longitudinal axis of said excitercoil; means for rotating said device in said bore hole; means fordetermining the ratio of said induced voltage in said sensor coil tosaid induced voltage from said reference coil as a function of theangular orientation of said device with respect to said ferrous element.2. A device in accordance with claim 1 wherein said reference coil iscoaxial with said exciter coil and said sensor coil is positioned at anangle to the axis of said exciter coil and said reference coil.
 3. Adevice in accordance with claim 1 wherein said reference coil and saidsensor coil are spaced from the axis of said exciter coil and each hasits axis parallel to the axis of said exciter coil.
 4. A device inaccordance with claim 1 wherein said reference coil and said sensor coilare each spaced from the axis of said exciter coil and wherein each hasits axis positioned at a like angle from said exciter coil axis.
 5. Anorientatable perforating device for subsurface use and adapted toperforate in a preselected orientation comprising, in combination:positioning means; rotating motor means attacHed to said positioningmeans; exciter coil means likewise attached to said positioning means,said exciter coil means producing an electromagnetic field which isaxially symmetrical with respect to said device when in an isotropicenvironment; receiver means longitudinally spaced from said exciter coilmeans and rotatable by said motor means, said receiver means comprisingan electromagnetic field sensor responsive to the magnitude and theconfiguration of said electromagnetic field, said sensor providing anelectrical signal indicative of said magnitude and said configuration;means for transmitting said signal to the surface; perforating meansactuatable from said surface and carried by said device and rotated bysaid motor means.
 6. A device in accordance with claim 5 in which saidreceiver means comprises a reference coil and a sensor coil, eachadapted for the production of an induced voltage from said field.
 7. Adevice in accordance with claim 5 in which said sensor is amagnetometer.
 8. An orientatable perforating device for subsurface useand adapted to perforate in a preselected orientation comprising, incombination, positioning means; rotating motor means attached to saidpositioning means; exciter coil means likewise attached to saidpositioning means, said exciter coil means producing an electromagneticfield which is axially symmetrical with respect to said device when inan isotropic environment; receiver coil means longitudinally spaced fromsaid exciter coil means and rotatable by said rotor means and includinga reference coil capable of producing an induced voltage from saidelectromagnetic field and a sensor coil means capable of producing aninduced voltage from said field, said sensor coil means having its axisoriented at an angle to the longitudinal axis of said device so thatwhen said field is axially non-symmetrical, the voltage induced in saidsensor coil varies with its relative orientation with respect to saidnon-symmetrical field; means responsive to the ratio of the inducedvoltage in said sensor coil to the induced voltage in said referencecoil and producing a signal indicative of said ratio which istransmitted to the surface; perforating means actuatable from saidsurface and carried by said device and rotated by said motor means.
 9. Amethod for orienting an actuatable subsurface device in a bore holecontaining a first casing string and a second casing string, to apreselected orientation comprising: positioning in said first casingstring a source of electromagnetic flux normally axially symmetric withsaid first casing string but asymmetrically distorted by the presence ofsaid second casing string; positioning a rotatable receiver meanslongitudinally spaced from said source within said first casing string,said receiver means being responsive to the magnitude and theconfiguration of said electromagnetic flux and providing an electricalsignal indicative of said configuration; transmitting said signal tosaid surface; rotating said receiver means so as to cause a registrationat said surface of its orientation with respect to said second casingstring; rotating said sub-surface actuatable device into saidpreselected position; subsequently actuating said device.
 10. The methodin accordance with claim 9 wherein said device is a perforator.
 11. Themethod in accordance with claim 9 wherein said source of electromagneticflux is placed in said second casing string, and said rotatable receivermeans is not necessarily longitudinally spaced from said source.
 12. Themethod in accordance with claim 9 wherein said receiver means comprisesa reference coil capable of producing an induced voltage from saidelectromagnetic flux and a sensor coil means capable of producing aninduced voltage from said flux, said sensor coil means having its axisoriented at an angle to the longitudinal axis of said casing string. 13.The method in accordance wIth claim 12 wherein said device is aperforator.
 14. The method in accordance with claim 9 wherein saidreceiver means is a magnetometer.
 15. The method in accordance withclaim 14 wherein said device is a perforator.